The method routinely used by clinicians to measure blood pressure in humans is well-known and requires the use of a sphygmomanometer, in combination with a stethoscope or, in the case of automated BP-measuring devices, electronic means to determine the Korotkoff sounds during the measurement. Whilst this method is non-invasive, it does not permit truly continuous monitoring of blood pressure in real time over prolonged periods providing, at best, intermittent blood pressure values.
Clinically, it is conventional to monitor the “mean arterial pressure” or MAP. The MAP is derived from the values for systolic and diastolic pressures according to the equation:MAP=Pd+⅓(Ps−Pd)                Where Pd=diastolic pressure and        Ps=systolic pressure        
There are many situations in which continuous monitoring of blood pressure is desirable, especially in operating theatres and in Intensive Care Units. The sphygmomanometer is not adequate in such situations, and conventionally an intra-arterial pressure catheter is used. However this is invasive and therefore not ideal. There is thus a need for an improved apparatus and method for non-invasive measurement of blood pressure, especially one which allows for substantially continuous monitoring to be made in a simple and non-labour intensive manner.
There have been many attempts to obtain a reliable estimate of arterial pressure using methods based on existing non-invasive tissue perfusion techniques: plethysmograms, photoplethysmograms (“PPG”), acoustic and optical Doppler techniques, CT/PET scanning methods, etc. using ever more advanced equipment and signal analysis (see e.g. http://en.wikipedia.org/wiki/Continuous_noninvasive_arterial_pressure).
Thus, whilst excellent clinical insights have been published no simple, practical solution to this problem has been developed. The main reason is that even though in theory, the rate of peripheral perfusion (that is, the volume of blood supplied, in millilitres, per 100 grams of perfused tissue, per minute) is proportional to the supplying blood pressure, in practice its measurement is far from simple due to the intrinsically complex anatomical and neurological autoregulatory nature of human tissue.
In purely mathematical terms, the multi-directional blood flow through the human skin's 3-dimensional and constantly (auto-)varying vessel diameters, opening/closing of anastomoses, etc. can best be described as a chaotic system. Further, the number and density of blood vessels varies greatly between different skin locations (e.g. there are over 140 capillary loops per mm2 on the forehead, but only about 40 per mm2 on the calf, [Ryan T J. The Physiology and Pathophysiology of the Skin New York Academic Press 1973, pp 571-651]). Dynamic measurements of single factors, e.g. blood volume or blood cell velocity can only at best give a “perfusion index”—which even then is fraught with uncertainty. Further, existing apparatus and methods use measurement sites (finger, wrist etc.) that are highly susceptible to movement artefacts.
The present invention aims to provide an apparatus and method which ameliorates or overcomes at least some of the problems experienced with the known apparatus and methods and to provide a simple, safe, inexpensive, yet reliable continuous non-invasive Mean Arterial Pressure (“cMAP”) measurement.